Discarded Area Increases Volume and Rate For Small Storm

[Groundwork Construction]

I am modelling a small project which is producing unexpected results. The flow path consists of a blacktop driveway. Tributary to the flow path is a small area of grass, some roof water, and of course the driveway. Post construction eliminates the grass area and all new impervious and existing roof water is directed to an infiltration bed. So, even with less catchment area tributary to the flow path I am still seeing an increase in both rate and volume for a 2 yr storm. All other storm events produce expected results with a slightly lower volume and peak rate. Please see attached files, Thanks

 

[psmart]

Since your calculations are using a weighted CN value, the area of grass causes provides a significant reduction in the composite CN value, from 98 to 77. At CN=77 your 2-year event barely exceeds the initial abstraction, so you actually get less runoff than the smaller area at CN=98. This is an expected result due to the non-linear nature of the SCS runoff equation and the use of a composite curve number. Please note there are other options for CN handling that you may want to consider as discussed at

http://www.hydrocad.net/curvenumber.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net